Dynamically creating a new instance of IList's type

Question

Hi, My application is processing IList's. ILists of different user defined types. I'm thinking that i can use reflection to to see what type of object the IList contains and then create a new instance of that type and subsequently add that to the IList itself?

So at any one time I might be processing

IList<Customer> l;

and I'd like to create a new instance of Customer

Customer c = new Customer(0, "None")

and then add that onto the list

l.Add(c);

Obviously doing this dynamically at run-time is the crux of the problem. Hope somebody can give me some pointers. Thanks brendan

Answer 1

The big problem here is: If you don't know the type, how do you know how to make a new one? Not every type in the world has a constructor that takes an int and a string.

Answer 2

Yes sorry i should have mentioned that the set of objects i will be processing will have a constructor that accepts an int and a string.

Answer 3

If you can use a parameterless constructor and set the properties afterwards then you can make your method generic, something like:-

    void Process<T>(IList<T> list, int x, string y) where T : MyBase, new()
    {
        T t = new T();
        t.X = x;
        t.Y = y;
        list.Add(t);
    }

Where MyBase is the base for your classes which expose the int and string properties. You can use an interface rather than a base class if you want.

Answer 4

You can use the Activator.CreateInstance method to invoke a constructor for a class via its type name (as a string) or an instance of System.Type.

Answer 5

Try this:

    public static void AddNewElement<T>(IList<T> l, int i, string s)
    {
        T obj = (T)Activator.CreateInstance(typeof(T), new object[] { i, s });
        l.Add(obj);
    }

Usage:

    IList<Customer> l = new List<Customer>();
    l.Add(new Customer(1,"Hi there ..."));

    AddNewElement(l, 0, "None");

(EDIT):

Try this then:

    public static void AddNewElement2(IList l, int i, string s)
    {
        if (l == null || l.Count == 0)
            throw new ArgumentNullException();
        object obj = Activator.CreateInstance(l[0].GetType(), new object[] { i, s });
        l.Add(obj);
    }

Answer 6

I think you should change your design. You can use abstract factory pattern. Using reflection would degrade performance.

Here is code for factory.

public abstract class MyStore {
    public abstract string Name { get; }
    public abstract void AddItem(int id, string name);
}

You can consider using interface if your abstract class has no code.

Then create Customer store.

public class CustomerStore : MyStore, IEnumerable<Customer> {
    List<Customer> list = new List<Customer>();

    public override string Name { get { return "Customer Store"; } }
    public override void AddItem(int id, string name) {
        list.Add(new Customer(id, name));
    }
    public IEnumerator<Customer> GetEnumerator() {
        return list.GetEnumerator();
    }
}

Usage

foreach (MyStore store in List<MyStore>)
    store.AddItem(0, "None");

If you want to consider type of store, use

switch (store.Name) {
case "Customer Store":
    SomeMethod((CustomerStore)store);
    break;
default:
    throw new WhatEverException();
}

Answer 7

You could use the Type.GetGenericArguments method to return the type argument of the generic type IList<T>. Then invoke the appropriate constructor.

  Type T = l.GetType ( ).GetGenericArguments ( ) [ 0 ];
  ConstructorInfo ctor = T.GetConstructor (
    new Type [ 2 ] { typeof ( int ), typeof ( string ) } );
  System.Diagnostics.Debug.Assert ( ctor != null );
  object instance = ctor.Invoke (
    new object [ 2 ] { 0, "None" } );